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A very loose concentration of young stars (perhaps 100 in a region several hundred light years across), conspicuous only because it consists of distinctive stars. An OB association consists predominantly of the most massive stars, of spectral types O and B; an R association consists of medium-mass stars, surrounded by reflection nebulae; and T associations are the birthplaces of the lowest-mass stars, in the form of T Tauri stars. All three types of association can be found together, and often have a star cluster within them. Like open clusters, associations are born from nebulae in the spiral arms of our Galaxy, but are so scattered that they disperse in 10 million years or so.